The present invention relates to polysaccharide derivatives containing aldehyde groups and to the acetal derivatives used in the preparation thereof. It also relates to a non-oxidative process for introducing aldehyde groups into polysaccharides. It further relates to the use of the cationic aldehyde-containing derivatives as paper additives.
As used herein, the term "paper" includes sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from natural sources as well as from synthetics such as polyamides, polyesters, and polyacrylic resins, and from material fibers such as asbestos and glass. In addition, papers, made from combinations of cellulosic and synthetic materials are applicable herein. Paperboard is also included within the broad term "paper".
Oxidative and non-oxidative methods have been used to introduce aldehyde groups into polysaccharides such as starches, gums, and celluloses. The oxidative methods used have included treatment with periodic acid, periodates, or alkali metal ferrates. See U.S. Pat. No. 3,086,969 (issued Apr. 23, 1963 to J. E. Slager) which discloses an improved process for the preparation of a dialdehyde polysaccharide (e.g., starch) using periodic acid; U.S. Pat. No. 3,062,652 (issued Nov. 6, 1962 to R. A. Jeffreys et al.) which discloses the preparation of dialdehyde gums (e.g., gum acacia, pectin, and guar) using periodate or periodic acid; and U.S. Pat. No. 3,632,802 (issued Jan. 4, 1972 to J. N. BeMiller et al.) which discloses a method for oxidizing a carbohydrate (e.g., starch or cellulose) with an alkali metal ferrate.
In the above methods the aldehyde groups are formed by the oxidation of the hydroxyl groups on the ring and/or side chain. Treatment with periodic acid or periodate selectively oxidizes the adjacent secondary hydroxyl groups on the ring carbon atoms (e.g., the 2,3-glycol structures), cleaves the ring, and results in a "so-called" dialdehyde derivative which is principally a hydrated hemialdal and intra- and intermolecular hemiacetals. Treatment of carbohydrates with alkali metal ferrates selectively oxidizes the primary alcohol group on the side chains without ring cleavage or oxidation of the ring hydroxyls.
The disadvantages of the oxidative method include degradation to lower molecular weight products and the formation of carboxyl groups due to further oxidation of the aldehyde groups. U.S. Pat. No. 3,553,193 (issued Jan. 5, 1973 to D. H. LeRoy et al.) describes a method for oxidizing starch using an alkali metal bromite or hypobromite under carefully controlled conditions. The resulting dialdehyde is reported to have a substantially greater proportion of carbonyl groups (i.e., aldehyde groups) than carboxyl groups. It also discloses a method for selectively oxidizing the side chains of starch derivatives (e.g., an alkoxylated starch such as dihydroxypropyl starch) under the same process conditions whereby the underivatized starch hydroxy groups on the rings are substantially non-oxidized.
The presence of carboxylic groups in aldehyde starches has several disadvantages in addition to the obvious reduction in the degree of aldehyde substitution. These include the introduction of hydrophilic properties due to the carboxyl groups, an upset in the cationic/anionic ratio when a cationic starch base is used (as in most papermaking wet end uses), and the possible formation of salts (in the above paper-making end use) which could give rise to ionic crosslinking.
The non-oxidative methods typically involve the reaction of the polysaccharide with an aldehyde-containing reagent. See U.S. Pat. No. 3,519,618 (issued July 7, 1970 to S. M. Parmerter) and U.S. Pat. No. 3,740,391 (issued June 19, 1973 to L. L. Williams et al.) which cover starch derivatives and U.S. Pat. No. 2,803,558 (issued Aug. 20, 1957 to G. D. Fronmuller) which covers a gum derivative. The starch derivative of Parmerter is prepared by reaction with an unsaturated aldehyde (e.g. acrolein) and has the structure Starch-O-CH(R.sup.1)-CH(R.sup.2)-CHO where R.sup.1 and R.sup.2 are hydrogen, lower alkyls or halogen. The starch derivative of Williams is prepared by reaction with acrylamide followed by reaction with glyoxal and has the structure ##STR3## The gum derivative of Fronmuller is prepared by treating the dry gum (e.g., locust bean or guar gum) with peracetic acid to reduce the viscosity, neutralizing, and then reacting with glyoxal. Water-soluble cellulose ethers (e.g., hydroxyethylcellulose) have also been reacted with glyoxal or ureaformaldehyde to give aldehyde-containing derivatives.
One of the disadvantages of introducing the aldehyde groups directly using an aldehyde-containing reagent is the possibility of the derivative crosslinking prior to use. This is a particular disadvantage when the products are being used to impart temporary wet strength to paper via a crosslinking reaction with the cellulose fibers. The Williams patent (cited above) alludes to this problem when it notes that solutions of the glyoxalated polymers "are stable for at least a week when diluted to 10% solids by weight and adjusted to pH 3" (see Col. 3, lines 60-63). The Parmerter patent notes that the starch aldehyde is "a substantially non-crosslinked granular starch derivative" and discusses the importance of the non-crosslinked character (see Col. 2, lines 40-45).
Therefore there is a need for aldehyde-containing polysaccharide derivatives and an improved non-oxidative method for their preparation which does not crosslink the derivative.